he nature of retroviral-like sequences in normal human genomic NA and their relationship to other endogenous retroviral equences have been studied by 1) DNA sequencing, 2) omputer-assisted analysis of nucleotide and deduced amino acid equences and 3) comparative analysis of different retroviral equences. A full-length clone (8.8 kb) of an endogenous human etrovirus was sequenced and found to have all of the tructural features and regulatory signals of a typical etrovirus. Nucleotide deletions and termination signals ender it nonfunctional. Preliminary sequencing of a new frican green monkey DNA clone suggests that a virus related to he human endogenous retrovirus has also infected monkeys. his retroviral DNA is not related to the HTLV lymphotropic lass of human retroviruses. e reported the first successful cloning of an infectious enotropic murine leukemia virus (MuLV). The retroviral env ene determines host range, patterns of viral interference, and pecific antigenicities. We have sequenced the env gene and ssociated regions (3 kb) of the NZB xenotropic provirus and ompared its nucleotide sequence with other classes of MuLVs. nalyses indicated a corresponding region in each type of MuLV ould encode for an oligopeptide that may be responsible for he biological properties noted above. It should be possible o test the model by constructing a recombinant in which the utative host range determinant sequence is replaced by the orresponding sequence from another type of provirus. ertain wild mice have a genetically determined resistance Fv-4) to infection by ecotropic MuLVs. We have molecularly loned and sequenced and endogenous pol-env-LTR segment that ppear to be responsible for this phenotype. Nucleotide equence analysis indicates that the Fv-4 determinant has eatures characteristic of an ecotropic rather than a enotropic env gene.